Flexographic printing elements are commonly used in printing, especially on surfaces which are soft and easily deformed such as packaging materials, including cardboard, plastic films, etc. Flexographic printing elements generally have resilient surfaces that are prepared from elastomeric photocurable resin compositions.
Photocurable (also known as photopolymerizable or photosensitive) resin compositions generally comprise an elastomeric binder (sometimes referred to as a prepolymer or an oligomer), at least one monomer, and a photoinitiator. To prepare relief image printing plate blanks, one or more layers of photocurable material are interposed between a support and one or more cover sheets, including slip and release films that protect the photosensitive surface.
Photocurable printing plate blanks are typically formed by well-known methods including solvent casting, hot pressing, calendaring, and extrusion. Printing plate blanks are then provided to the print shop customer who processes the printing plate blanks through a variety of steps to produce relief image printing element products having the desired properties. Various examples of manufacturing processes can be found in U.S. Pat. No. 5,135,827 to Bohm et al., in U.S. Pat. No. 5,735,983 to Goss et al., and in U.S. Pat. No. 4,622,088 to Min, the subject matter of each of which is herein incorporated by reference in their entirety.
FIG. 3 depicts a typical prior art manufacturing process for producing photosensitive printing plate blanks. A photocurable resin composition is extruded through an extrusion die 40 and is calendared between two flexible supports. The printing blank is then cooled (e.g., to a temperature of about 170° F.) and processed through a thickness profiler 42. Thereafter, the printing plate blank is processed in a forced air cooling device 50 and is transported on a conveyor. Printing plate blanks are then cut to the desired size and are provided to a print shop customer for processing to make the desired relief image printing element.
In the print shop, processing steps for forming relief image printing elements typically include the following:                1) Image generation, which may be mask ablation for digital “computer to plate” plates or negative production for conventional analog plates;        2) Back exposure to create a floor layer in the photocurable layer and establish the depth of relief;        3) Face exposure through the mask or negative to selectively crosslink and cure portions of the photocurable layer not covered by the mask or negative, thereby creating the relief image;        4) Developing to remove unexposed photopolymer by solvent (including water) or thermal development; and        5) If necessary, post exposure and detackification.        
Removable coversheets are typically provided to protect the photocurable printing element from damage during transport and handling. Prior to processing the printing elements, the coversheet(s) are removed and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer (or thermal development) removes the unexposed areas of the photopolymerizable layer, leaving a printing relief that can be used for flexographic printing.
Back exposure is a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to establish the depth of relief and establish greater support.
Following this brief exposure step (brief as compared to the imagewise exposure step which follows), an imagewise exposure is accomplished utilizing a photographic negative mask or a digitally-imaged mask, which is in contact with the photocurable layer and through which actinic radiation is directed.
The type of radiation used is dependent on the type of photoinitiator in the photopolymerizable layer. The photographic negative or digitally-imaged mask prevents the material beneath its opaque areas from being exposed to the actinic radiation and hence those areas covered by the opaque areas of the mask do not polymerize. The areas not covered by the opaque areas of the mask are exposed to actinic radiation and polymerize. Any conventional sources of actinic radiation can be used for this exposure step. Examples of suitable visible or UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units and photographic flood lamps.
Streamlining workflow is a huge desire of most print shop customers involved in the processing of photosensitive printing elements in order to provide more efficient and faster processing of the photosensitive printing elements and to increase productivity.
Furthermore, achieving consistent floors and back exposure times for flexographic printing elements, especially thin plates, is difficult for both print shop customers and printing plate manufacturers. After the manufacturer makes the printing plate blanks as described herein, the floor of the printing plate is set in the print shop by the print shop customer using back exposure. This back exposure is subject to variation due to many factors including UV-light source variation, temperature, age of the plates, and raw material variation, which can impact the consistency of the floor layer thus produced.
One of the difficulties print shops face in achieving consistent back exposure is described in U.S. Pat. No. 4,927,723 to Cusdin, the subject matter of which is herein incorporated by reference in its entirety. When preparing thin flexographic printing plates (e.g., 2 to 3 mm overall thickness), the cured floor thickness forms a substantial proportion of the thickness of the finished plate. The back exposure needed to produce the cured floor over the entire plate gives the appropriate floor depth in areas where the image-forming negative is predominantly transparent, but in small opaque areas of the negative, corresponding to “shadow reverses” of the printing plate, there is a tendency for the reverses to fill in because the back exposure operation tends to negate the effect of the masking of the fine opaque area of the negative and thus to cure the polymer in what should correspond to a small uncured zone of the printing plate. If the back exposure operation is correspondingly reduced in order to avoid this tendency, the stability of the highlight dots in other parts of the plate can be jeopardized through the formation of an excessively thin floor to the relief plate.
The '723 patent attempts to solve this problem by using a positive of the image as a mask for the back exposure step to ensure that, in those areas of the plate where delicate printing relief areas (for example highlight dots) are to arise, there is a substantial floor in order to stabilize the highlight dots, whereas in other areas of the plate (where the relief printing areas predominate) the floor thickness formed through back exposure is correspondingly reduced to preserve the openness of the fine uncured “shadow reverses” through the use of a part of the positive which is predominantly opaque as the mask for the back exposure operation. However, this process adds an additional step to the plate making process resulting in additional time to create the printing element.
The inventors of the present invention have determined that it would be highly desirable to streamline the workflow of the print shop customer while achieving more consistent floor layers in relief image printing elements. To that end, the inventors of the present invention have discovered that it is possible to produce a printing plate blank having a precured floor layer formed therein which is then delivered to the print shop customer for further processing.
Achieving a pre-cured floor eliminates a significant number of quality problems. Providing a pre-cured floor also allows the use of a simpler polyethylene terephthalate (PET) backing, without the need for an adhesive so long as the adhesion of the cured photopolymer on the “raw” PET would be sufficient.
Creating a plate with a pre-cured floor offers the print shop customer two potential benefits:                1) increased productivity due to elimination of a processing step because back exposure step is not required. Elimination of the back exposure step eliminates set-up, ongoing labor costs and periodic adjustment of the equipment for the print shop customer; and        2) extremely uniform and consistent floor layers in processed printing plates.        
U.S. Pat. No. 6,759,175 to Daems et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method for on-site preparation of a relief image comprising the steps of: (a) laminating a material comprising: a first peelable support, an image recording layer and an adhesive layer onto a UV-sensitive material comprising a support, an UV-sensitive layer, wherein the adhesive layer is laminated to the UV-sensitive layer; (b) image-wise exposing the image recording layer to form a mask; (c) flood exposing the UV-sensitive material through the mask; and (d) developing the UV-sensitive material. The peelable support is removed either before step (b), (c) or (d). As a result the extent of monomer diffusion from the UV-sensitive layer to the image recording layer is reduced and the adhesion between the image recording layer and the UV-sensitive material is optimized. However, Daems does not address the issue of variations in the floor layer and factors such as UV light source variation, temperature, and raw material variation are still an issue.
U.S. Patent Application Publication No. 2005/0015297 to Stebani et al., the subject matter of which is herein incorporated by reference in its entirety describes a method of marketing photopolymeric sleeves for flexographic printing in which a customer is supplied with a suitable apparatus and know-how for producing photopolymeric sleeves so that the customer is enabled to produce photopolymeric sleeves himself. However, Stebani et al. is limited to the production of printing sleeves and also does not address factors such as UV-light source variation, temperature, and raw material variation.
The advantage of the present invention lies in standardizing the floor layer thickness, which serves to prevent variation in the thickness of the floor. Another advantage lies in the cost savings to the customer, because the pre-cured floor layer enables the workflow to be accomplished more quickly by reducing a step in the process, and eliminating set-up, ongoing labor costs, and periodic adjustments associated with the back exposure step. Accordingly, less time and less machinery are required to produce the finished relief image printing element.